Field of the Invention
The invention relates to an optical transmitter, and a control method of an optical transmitter, and more particularly to an optical transmitter including an optical modulation unit that modulates an incident light beam, a modulation signal drive unit that inputs a modulation signal to the optical modulation unit, a controller that controls the optical modulation unit and the modulation signal drive unit.
Description of the Background Art
It is known that the optimum bias voltage of an MZ (Mach-Zehnder) optical modulator drifts by temperature or aging change. Therefore, in order to keep the quality of a transmission optical signal, control for making a bias voltage to follow an optimum bias value is performed. For example, in an optical transmitter that performs ABC (automatic bias control) of an I/Q modulator, there is known an optical transmitter that controls three bias voltages, I-ch, Q-ch, and Phase in turn in a state where an arbitrary electric waveform is input, superimposes low-frequency signals (Dither) on the bias voltages of I-ch and Q-ch at this time, and performs feedback control to a convergence point where an error signal of Dither detected from a monitor PD (Photo-Diode) current becomes 0.
In an optical transmitter disclosed in Japanese Patent Application Laid-Open No. 2012-217127, as a method of searching initial values of respective bias voltages to be applied to first and second mach-zehnder optical modulators when the CW (Continuous Wave) optical output of an optical modulation unit is the most extinct, each a total of three points of the respective bias voltages of the first and second mach-zehnder optical modulators, and the bias voltage of an optical phase regulator are each changed in a range of ±Vπ, and an optical output level is monitored and determined. Only a combined light beam of the first and second mach-zehnder optical modulators can be monitored, and therefore there is a possibility that even when the combined light beam is in an extinction state, output light beams from the first mach-zehnder optical modulator and the second mach-zehnder optical modulator are mutually opposite in phase. Therefore, it is difficult to determine whether the output light beam from each of the first and second mach-zehnder optical modulators is in the extinction state. Therefore, in a system disclosed in Japanese Patent Application Laid-Open No. 2012-217127, an optimum bias voltage is determined by the combination of the total of the three points of the respective bias voltages of the first and second mach-zehnder optical modulators, and the bias voltage of the optical phase regulator. Accordingly, there is a problem that it takes a long time.